Reviewing the genetic evidence for sex-biased dispersal in elasmobranchs

Dispersal in many organisms is asymmetric by sex, a pattern that is often identified through the use of genetic tools. Sex-biased dispersal (SBD) is thought to derive from the varying fitness needs of females and males, as mediated by local ecology and life history. SBD is frequently reported in elasmobranchs (sharks, rays), long-lived fishes that often give live birth to well-developed young and are capable of dispersing thousands of kilometers. While many studies point to male-biased dispersal (MBD) being common, results are highly variable and no clear trends have yet emerged, even as the number of case studies has grown over the past decade. Here, we evaluated patterns in sampling regime, molecular marker type, and analysis method for every genetic structure study published to date that allowed for an assessment of SBD in elasmobranchs. We find that while some degree of MBD in elasmobranchs is likely, factors such as the pooling of life stages during data analysis and the inherent characteristics of different marker types, may lead to an overemphasis on male dispersal and potentially obscure genetic signals of female and male reproductive philopatry. The role of life history and biogeography in determining patterns of SBD in sharks and rays is also discussed.